1. Field of the Invention
Considerable research and development effort currently is under way in an effort to develop commercially practical processes for upgrading coals (bituminous, subbituminous and lignite) to provide a more ecologically acceptable product, that is, a product having both a low sulfur and a low ash content. However, many of the current conversion processes suffer from inadequate separations of mineral matter (sulfur containing ash values).
The present invention relates generally to coal deashing processes and, more particularly, but not by way of limitation, to improved recovery and utilization of coal liquefaction products in coal deashing processes.
2. Description of the Prior Art
Various processes, including gasification, liquefaction, pyrolysis and combinations of these processes have been investigated. One promising process is referred to as the Solvent Refined Coal process. This process includes solvent extraction and comprises dissolving ground coal in solvent, in the presence of hydrogen, to provide a mixture comprising (1) dissolved coal, (2) solid, insoluble coal values and (3) solid, insoluble mineral matter of ash. Separation of the dissolved coal from the solid, insoluble materials has proved to be arduous and costly when attempted using either filtration devices or hydroclones, for example.
Moreover, even when such separation attempts have been successful, it has been found that the recovered dissolved coal frequently has an ash content considerably higher than desired. An ash content of 0.16 weight percent or less in the dissolved coal is desirable since, with such a low ash level, it is possible to burn such low-ash containing dissolved coal in electric power plants or the like without the necessity of installing costly fly-ash removal equipment for the stack-gases. Further, a low sulfur content is desired since this precludes the need to install sulfur abatement devices such as stack gas units to remove sulfur dioxide.
It also has been found that when filters or hydroclones are used as separation devices that an undesirably large amount of dissolved coal may remain either in the filter cake or hydroclone underflow with the separated ash. It has been suggested that such filter cake or hydroclone underflow be used as a feed for a hydrogen generation plant because of the carbon content of the contained coal values. This is uneconomical, however, because of the relatively high content of dissolved coal.
Many other prior art separation processes have been found to be impractical. Some depend upon the utilization of unusual or especially prepared coal dissolving solvents, see for Example U.S. Pat. No. 3,867,275. Further, such dissolving solvents are expensive since they frequently are generated from various high cost petroleum fractions rather than being derived from lower cost coal. Clearly, solvent derived from coal is preferred to that derived from petroleum.
Other prior art processes have proved to be economically unsuitable because they require treatment of the entire dissolver effluent with a second liquid intended to enhance or promote separation of undesirable solid materials from the dissolved coal. One disadvantage of such processes lies in the fact that large volumes of dissolver effluent have to be treated.
In one prior art process, a special kerosene fraction is prepared via a controlled hydrogenation step, see U.S. Pat. No. 3,852,182; 3,852,183 and 3,856,675. Use of such solvents requires relatively long settling rates.
Another process, U.S. Pat. No. 3,791,956, requires the use of solvents such as n-decane, cyclohexane or decalin, all of which are expensive and result in processes which do not appear to be economical.
Successful processes for the deashing of coal utilizing critical solvent techniques are disclosed in U.S. Pats. Nos. 3,607,716 and 3,607,717, which patents are assigned to the same assignee as the present invention. In one such process, following the initial dissolution of particulate coal in suitable solvents, in the presence of hydrogen and at elevated temperatures and pressures, gaseous products and very light hydrocarbon fractions are separated from the dissolver effluent which then is advanced directly to a vacuum stripping tower. Overheads from that tower can be fractionated to separate contained light hydrocarbons from the solvent. The light hydrocarbons may be utilized for known purposes and the solvent can be recycled to the initial dissolution step.
Still bottoms from the vacuum tower, containing the mineral matter, then are advanced to a critical solvent deashing section. In that section, the vacuum still bottoms are mixed with a light organic "deashing" solvent. Thereafter, under suitable conditions of elevated temperature and pressure, a first stage ash concentrate fraction is separated. Subsequently, at even higher temperatures but at essentially constant pressure, deashed coal is recovered from a second stage.
It has been determined that under certain critical solvent deashing operating conditions, in the deashing section, that the first stage ash concentrate contains a significant quantity of dissolved coal values. Generally, it would be desirable to use the ash concentrate as a feed to a hydrogen plant. However, this is uneconomical because of the quantity of soluble coal values present in the ash concentrate.